The risk to the financial system most cited by respondents in the
2017 H2 survey was "UK political risk," mentioned by 91% of
respondents and up 9 percentage points since 2017 H1.

Political risk was also overwhelmingly cited as the number one
source of risk, by 67% of the 96 respondents, compared to
geopolitics (7%), cyber attack (7%) and the risk of a UK economic
downturn (3%).

Here's the chart:

Bank of
England

The biannual survey asks market participants about perceived
risks to, and their confidence in, the stability of the UK's
financial system. Respondents tend to be executives responsible
for firms' risk management or treasury functions.

Around 90% of those respondents that cited political risk
explicitly referred to the implications of the Brexit vote.
However, cyber attack was the risk most cited by UK building
societies and large UK banks, while political risk was most cited
by other sectors.

A spike in the perception of the threat posed by UK politics was
recorded in the first survey of 2016, carried out just
before the Brexit vote.

At that time, the proportion of respondents that cited this
threat
jumped from 26% to 72%, and almost all respondents that
mentioned this risk referenced the possibility of the UK leaving
the European Union.

The second and third most commonly cited threats to the UK
financial system in 2017 H2 were geopolitical risk (61%,
unchanged from 2017 H1) and cyber attack (57%, up six percentage
points). The proportion of respondents fearing cyber attacks is
now at its highest level since the survey began in 2008, while
the proportion of respondents citing "sovereign risk" has dropped
to 14%, from 76% in 2011 H1.

Compared to 2017 H1, a slightly larger proportion of respondents
cited the risk of a UK economic downturn (34%, up six percentage
points).